LAUSR

Self‐assembled nanocrystal superlattices represent an emergent class of designer materials with potentially programmable functionalities. The ability to construct hierarchically structured nanocrystal superlattices with tailored geometry and porosity is critical for extending their applications. Here, 2D superlattices comprising monolayer FeP nanoframes are synthesized through a space‐confined topochemical transformation approach induced by the Kirkendall effect, using carbon‐coated Fe3O4 nanocube superlattices as a precursor. The particle shape and the close‐packed nature of Fe3O4 nanocubes as well as the interconnected carbon layer network contribute to the topochemical transformation process. The resulting 2D FeP nanoframe superlattices possess several unique and advantageous structural features that are unavailable in conventional 3D nanocrystal superlattices, which make them particularly attractive for catalytic applications. As a proof of concept, such 2D FeP nanoframe superlattices are harnessed as highly efficient and durable electrocatalysts for the hydrogen evolution reaction, the performance of which is superior to that of most FeP‐based catalysts reported previously. This topochemical transformation approach is scalable and general, representing a new route of designing hierarchical superlattices with highly open features that cannot be accessible by traditional self‐assembly methods.